For high power semiconductor switches, device theory, Y. C. Kao and D. J. Page, "Theoretical Limits for High Voltage Rectifiers and Thyristors, " IEEE-IEDM, Washington, DC (1979), predicts that a scaling up of the blocking voltage capability leads to higher conduction losses and/or longer switching times. The required increase of base width with voltage results in a higher forward voltage drop, hence the higher conduction loss. Alternatively, the increase in forward voltage drop in the high power semiconductor switch may be minimized by increasing the minority-carrier lifetime in the base region. The higher minority-carrier lifetime, also, leads to a longer switching time, aggravating the increase in switching time already resulting from the wider base (i.e., additional stored charge).
To illustrate this point, switching transistors serve as a prime example. Commonly, high power switching transistors are capable of operating in the temperature range between -65 to 200.degree. C. For instance, Powerex Transistor D60T 2N6841 is rated at a breakdown voltage between the collector and emitter with the base opencircuited (BV.sub.CEO) and at a breakdown voltage between the collector and base with the emitter open circuited (BV.sub.CBO) of 450 volts and 750 volts, respectively. At a collector current (I.sub.C) of 50 amperes and gain (h.sub.FE) of 10, the voltage between the collector and the emitter (V.sub.CE) is 2.5 volts. However, Powerex Transistor D60T 4005, which has the same emitter and collector structure as previously mentioned Powerex transistor D60T 2N6841, is rated at BV.sub.CEO and BV.sub.CBO of 750 volts and 1150 volts, respectively. The voltage drop (V.sub.CE) at I.sub.C =20 amperes and h.sub.FE =10 is 5.0 volts. An increase of BV.sub.CEO by a factor of 1.7 leads to an increase of V.sub.CE by a factor of 2 and a decrease of I.sub.C by a factor of 2.5. The increase in V.sub.CE contributes both to the conduction and the switching losses in the switching transistors.
To improve the performance of high power semiconductor switches, innovative solutions are needed to increase the blocking voltage without sacrificing either the conduction loss or the switching speed.